The invention relates to a pulp dryer screen assembly and a method for tightly mounting the screen on a frame.
Wood pulp is conventionally produced in a continuous web which is subsequently dried in a pulp dryer. In one type of dryer the web or pulp sheet is carried through the dryer between hot air streams which are discharged from blow boxes above and below the web. The air blown onto the web is heated, typically by steam heating coils having finned surfaces. The air picks up wood pulp fibers in the process which would tend to accumulate on the finned surfaces thereby appreciably reducing their efficiency.
For this reason, filtering screens are conventionally placed over the face of steam heating coils to collect the fibers and avoid clogging the coils. A large number of screens is required, and sometimes several hundred screens can be required for a dryer. The most common type of screen is rectangular and is placed over a rectangular perimeter frame. One type of conventional screen assembly employs a perimeter frame of square tubing. The screen is fitted over two sides of the frame and at both the ends. An angle section clamps the screen against the sides and the ends of the screen. Rivets or screws are typically used to secure the angle section to the frame itself. It may be appreciated that a pronounced inner edge occurs about the frame where the angle section fits over the flat surface of the screen.
Eventually such screens become clogged with fibers from the pulp which are desirably removed to improve efficiency of the dryer. This is usually done by vacuuming the screens at least daily. A heavy-duty vacuum cleaner is employed and the nozzle of the cleaner is brushed over the screens to remove the fibers. During this vacuuming operation, the nozzle of the vacuum cleaner brushes over the pronounced inner edge of the angle sections which overlie the screens. This often causes the nozzle to snag the angle sections, sometimes unseating the screens when the operator attempts to unsnag the nozzle from the screens. The screens sometimes fall off the hot coils and have to be retrieved and rehung. Often the screens are not retrieved and rehung while the dryer is in operation due to the undesirability of exposing persons to the very hot atmosphere in the dryer (e.g. 212.degree. F. or 100.degree. c. at the access door). Thus the screens may remain off of the coils for days or weeks until the dryer is cooled during breaks in production.
Repeated snagging can often bend the angle sections, thus increasing the likelihood of snagging again in future. Furthermore, abrasion between the nozzle and the screens sharpens the nozzle which catches the angle sections, leading to damage to the screens during the vacuuming operation. Broken areas of the screens around the frame lead to corresponding areas on the coils which accumulate pulp fibers. The wear and tear on screens due to the cleaning operation and dislodging of the screens to loosen on their frames.
The screens are conventionally provided with crossbars extending between frame members to support the frame members against deflection due to screen tension. The cross bars are conventionally straight and rest close to the backs of the screens. The tension in the screens often decreases rapidly and the resulting slackened screen can bend over the crossbars and frame members. This slackness also leads to additional screen wear and breakage of the screens adjacent the crossbars and frame edges during the vacuuming operation. Eventually, many of the frames become twisted and do not seal against the mounting frames provided adjacent to the coils. A poor seal permits wood fibres to pass edges of the screens and this also causes the fins of the coils to become clogged with wood fibers.
A pulp drier is the only machine in a pulp mill that has to be continuously cleaned during operation of the mill, to enable the mill to continue to operate. The work has to be done manually in high heat and high humidity conditions. It is important that the screens can be quickly cleaned, because the operators are subjected to heat fatigue, and are required to have rest periods between relatively short work periods.
Therefore, there is a significant demand for a screen which is easier to vacuum and which remains taut during the cleaning operation. It would also be desirable to produce a frame which is rigid enough and holds the screen taut. Loose screens are more difficult to clean because fine fibres tend to tangle between loose weaves of the screen, which increases the time required to clean the screen. Furthermore, a simple means of securing the screen to adjacent framework is desirable, so as to reduce the risk of the screen falling off the supports during cleaning.